The present invention relates to wear resistant coatings for substrates and more particularly to such coatings which resist the erosion effects of particles impacting the coated substrate.
One of the major causes of outages for steam turbines used in generating electricity is due to material failure resulting from the combination of erosion, corrosion, stress, fatigue and creep. These failures are induced by the atmosphere and temperature inside the turbine and the velocity of the turbine blades. Superheated steam circulating in the system can reach 540 degrees Celsius. The amounts of chloride, sulfate and nitrate ions in the steam are often controlled, but concentrations of these ions on the order of one to ten parts per billion can give rise to concentrations of over ten weight percent in the steam condensate.
According to data published by the Electric Power Research Institute solid particle erosion of turbine blades accounts for 13.7 percent of electric utilities repair expenditures and water corrosion accounts for an additional 10.8 percent. Erosion is defined as the loss of material from a solid surface due to relative motion in contact with a fluid that contains particles. The phenomenon occurs as a result of different mechanisms depending upon the composition, size and shape of the eroding particles, their velocity and angle at impact, and the composition and microstructure of the surface being eroded.
Erosion of turbine blades occurs for many reasons. Particles are often generated as oxide scale which exfoliates from the boiler and steam conduits. These particles are carried along by the steam flow and strike the rapidly turning turbine blades. Erosion of a surface can also occur without the presence of solid particles via mechanisms of liquid erosion. For example, if there is a formation and subsequent collapse of bubbles within the liquid, cavitation erosion occurs. Cavitation damage has been observed in hydraulic pumps and turbines as well as other systems where the high speed flow of liquid causes local hydrodynamic pressures to vary widely and rapidly. This type of erosion is often evidenced on the blades in the low pressure end of large steam turbines where the components are exposed to high velocity steam containing moisture droplets.
In order to provide turbine blades which resist corrosion and erosion, the blades are generally fabricated from stainless steel because of its erosion and corrosion resistance, as well as its other mechanical properties such as creep and fatigue resistance. The tips of previous blades were sometimes fitted with an insert formed of a Stellite Haynes (Trademark) alloy to further enhance the wear resistance of the blade.